State-of-the-art mechanical gyros are available for most uses, but their high cost, long start up time, low reliability and problems associated with acceleration make implementation of solid state inertial sensitive systems such as optical gyros attractive. To be acceptable for a broad range of products, optical gyros must have a large dynamic range. For example, a gyro that is to be used in an inertial navigation system for a manned aircraft, sufficiently accurate to accomplish the functions of navigation, control and support of weapons delivery, and reconnaissance, should be capable of sensing a peak angular input rate of about 400.degree./sec. On the low end, its bias instability should be better than 0.01.degree./hr. This represents about eight orders of magnitude dynamic range and is a primary navigational grade optical gyro performance objective. Secondary, but still important performance goals are: scale factor errors consisting of scale factor stability, scale factor asymmetry and scale factor linearity of not more than 10 ppm; a sensitivity to acceleration which is not greater than 0.01.degree./hr/g; an allowable operating temperature of from -30.degree. F. to 160.degree. F.; axis alignment instability of not more than 10 arc seconds; and the ability to withstand linear accelerations up to 10 g.
Concepts for inertial sensing based on ring lasers have thus far exhibited the best performance. For example, a ring laser gyro is available which employs a mechanical method known as dithering to circumvent the lock-in problem characteristic of ring laser gyros at low rotation rates. Although this gyro has exhibited the best performance of those gyros presently available, the use of mechanical dithering forsakes the potential advantages of a genuine solid state system. This prior art gyro is also large and costly since it has unitized construction and requires ultra high quality optics. The other presently available ring laser gyro is based on a magneto-optic mirror which acts as an electronic bias away from the lock-in zone, allowing low rotation rate operation. It is smaller and less costly than the previously mentioned mechancially dithered ring gyro. However, its performance is not as good, its cost remains high and its prospect for cost reduction due to production economics appears low.
Other laser gyros have been proposed including ones wherein effort has been made to introduce a Faraday rotator as a bias element into the ring laser cavity. However, the extreme thermal and magnetic sensitivity of such a device makes it questionable as to whether it will ever be developed far enough to meet the performance requirements outlined above. There are also investigators in the art presently pursuing a multioscillator (four mode) ring laser concept based on the use of a quartz crystal to split right and left hand circularly polarized modes in frequency. Each of these modes is split once again by a Faraday element. Each set of circularly polarized light beams is then mixed to obtain a frequency output dependent upon rotation rate. While the multioscillator approach differs from the two commercially available gyros mentioned above, there does not appear to be any reason to expect substantial reductions in size and cost by adopting a system that is more complex in terms of optical components and eventual readout. It is unlikely in the future that any of the aforementioned optical gyros or concepts will meet the desirable attributes of having high accuracy, small size, and low cost.
U.S. Pat. No. 3,879,310 by Greenstein discloses a ring laser gyro based on a saturable absorber gas element within the ring cavity which offers potential advantages in that outstanding bias stability can be achieved through the action of the saturable absorber gas. Also, operation in the preferred 3.39 m line of helium-neon, which has extremely high gain, results in a potentially smaller, high performance ring laser gyro. Although Greenstein's ring laser gyro has potential competitive advantages over those currently in development, it suffers many of the same problems which characterize prior art ring laser gyros, namely long development time, the need for high grade and consequently expensive optics, and a fundamental size limitation due to the gain of the neon gas lasing medium.
Passive cavity laser gyro configurations unlike ring laser gyros have only recently been investigated primarily due to the late availability of single mode fibers of reasonably low attenuation. One such device utilizes the difference in bandpass generated by an etalon measured in two different directions by counterpropagating beams. Its shortcoming is it cannot meet the navigational grade gyro requirements because of limited q (or Finesse) in real etalons. With this last gyro as an exception, all known actively researched efforts in passive cavity laser gyros use counterpropagating beams which pass through a single mode fiber coil.
Some investigators have demonstrated that a fringe pattern can be generated through the mixing of two counterpropagating beams in a single mode fiber-optic coil. Rotational motion of the fiber coil results in a phase shift between the two beams and a consequent change in the intensity of the central fringe. One device uses relative intensity measurements to determine rotation rate and since it is difficult to make measurements of intensity to much better than 0.01%, the dynamic range of such a device is quite limited. Efforts by other groups have involved the development of means whereby the ability to read out the phase difference of counterpropagating beams can be enhanced. However, they are still fundamentally limited in dynamic range because they basically are analog measuring devices.
Phase nulling fiber-optic laser gyro schemes have been disclosed in the the article "Phase-Nulling Fiber Optic Laser Gyro" Optic Letters, Volume 4, Page 93, March 1979, by R. F. Cahill and E. Udd, "Solid-State Phase Nulling Optical Gyro", Applied Optics, Volume 19, Page 3054, 15 Sept. 1980, by R. F. Cahill and E. Udd, and "Techniques for Shot-Noise-Limited-Inertial Rotation Measurement Using a Multiturn Fiber Sagnac Interferometer" by J. L. Davis and S. Ezekiel published 13 Dec. 1978 in SPIE Volume 157, Laser Inertial Rotation Sensors. Those papers disclose early Phase-Nulling Fiber Optic Gyros which, although were shown to operate, had not yet been involved in the intensive development required to bring a concept to practical use, so that the problems inherent in ring laser gyros can be bypassed by adopting passive cavity techniques in applying fiber optics and avoiding the low sensitivity nonlinear, analog output, which otherwise limits their dynamic range to a much lower level than that achieved by ring laser gyros.
The previously disclosed Cahill and Udd gyro is a linear rotation sensor rather than sinusoidal sensor. It produces an inherently digital output via a frequency change proportional to rotation rate. This gyro uses the nonreciprocal phase shift resulting from an induced frequency difference between counterpropagating beams in a fiber-optic coil to null out nonreciprocal phase shifts due to rotation. Thus it has the potential for wide dynamic range, high sensitivity and linear rotation sensing. It also has an inherently digital output desirable for modern guidance systems. The present invention teaches various improved embodiments of this gyro.
From this brief overview of the prior art devices, two major conclusions can be drawn. Firstly, ring laser gyros which have been under development for eighteen years probably will not undergo substantial reductions in size or cost in the foreseeable future, and secondly, passive cavity laser gyros and in particular those based on fiber optics, offer little hope of obtaining the performance levels of the existing ring laser gyros without an inventive breakthrough. What has been required is a low-cost, solid-state laser gyro with a wide dynamic range capability so that in the long term its cost and size allows it to be a replacement for not only the high quality gyros presently required in inertial guidance systems but ultimately for all purposes in which an electrical inertia indicating signal is required.